Radio wave absorbing panel

ABSTRACT

A radio wave absorbing panel is made up of a first insulating substrate and a second insulating substrate disposed in parallel a predetermined distance apart and a middle insulating substrate disposed between and parallel with the first and second substrates. The first and second substrates each have a conducting film coated over the entire surface of one side. On one side of the middle insulating substrate are coated multiple conducting films disposed in the form of stripes or a matrix. By this means a thin radio wave absorbing panel having excellent radio wave absorption and transparency to light can be obtained.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to a radio wave absorbingpanel used for absorbing radio waves incident on a building or radiowaves produced by OA equipment in an office and, more particularly, to aradio wave absorbing panel which both absorbs incident radio waves orOA-equipment-generated radio waves and transmits visible light.

[0003] 2. Description of the Related Art

[0004] In recent years, along with increases in numbers of high-risebuildings, cases of radio waves of TV frequency bands such as VHF andUHF being reflected by buildings have become more common. Consequently,ghosting, which arises on a TV screen when radio waves arriving at theantenna of a TV receiver directly from a TV station (direct waves) andradio waves reflected by buildings (reflected waves) are incident on theantenna simultaneously, has become a serious social problem.

[0005] Use of wireless LANs (local area networks) in offices is rapidlyspreading. Popularization of such wireless LANs has led to the expansionof the frequency band of radio waves used in offices to 1-6 GHz. It isthus expected that in-office radio wave environments will becomedeteriorated due to interferences by electromagnetic noises produced byvarious closely positioned OA equipment. This makes it necessary toreduce undesired radio waves in offices by using building materialsprovided in those offices.

[0006] For these reasons, with the object of reducing the number of theradio waves reflected by exterior walls of buildings or the radio wavesgenerated in offices, radio wave absorbing panels made of magneticmaterials such as ferrite have been affixed to or embedded in exteriorwalls or inside building materials to absorb those radio waves.

[0007] For window glass installed in building window openings, to reduceair-conditioning cooling loads in summer (to save energy), glass coatedwith a film having a heat ray reflecting function has been used;however, because films having a high heat ray reflecting capability havelow electrical resistance, their reflectivity of radio waves is high andthey are a cause of radio wave obstruction.

[0008] Radio wave absorbing panels for reducing radio wave reflection inwhich ferrite is used cannot be applied to window openings of buildingsbecause ferrite does not transmit visible light. Consequently thesituation has been that heat ray reflecting ability has been sacrificedand transparent heat ray reflecting films having relatively highelectrical resistance have been coated on windows of buildings to reduceradio wave reflection and prevent radio wave obstruction by transmittingradio waves through window openings into buildings.

[0009] Concerning window glass for buildings and vehicles, technology isknown (from, for example, Japanese Patent Laid-Open Publication Nos. HEI3-250797, HEI 5-042623, HEI 5-050548 and HEI 7-242441) whereby, with theobject of preventing obstruction due to radio wave reflection ofhigh-performance heat ray reflecting films, high heat ray reflectivityand low radio wave reflectivity are realized at the same time by aconducting film being divided up into areas of a size amply smaller thanthe wavelength of incident radio waves to raise its radio wavetransmittivity.

[0010] Radio wave absorbing panels in the related art which have radiowave transmittivity are an attempt to prevent radio wave reflectionproblems by providing window glass of buildings with radio wavetransmittivity; however, associated with these there are the problemsthat incoming radio waves penetrate to the inside of the building andaffect office equipment such as personal computers and thatelectromagnetic waves radiated from electronic appliances inside thebuilding leak through the window glass to outside the building. Althoughthis kind of radio wave obstruction is expected to increase in thefuture, no effective countermeasure has been taken besides reflectingand thereby blocking radio waves by using a conducting wire mesh or aconducting film on windows of buildings, and in districts where there isa likelihood of a radio wave reflection obstruction arising it has beendifficult to build buildings which have large-area windows and blockradio waves.

[0011] To solve this problem, it is necessary to create a practicallyusable transparent panel which absorbs radio waves instead of reflectingor transmitting them.

[0012] At present there are radio wave absorbing panels made bydisposing in parallel two transparent substrates each coated with aconducting film having a controlled sheet resistivity, with which panelsit is possible to realize a high radio wave absorbing capacity byutilizing resonance caused by interference of multiple reflections ofradio waves. However, to absorb VHF band (about 100 MHz) radio waves,the gap between the two substrates constituting the radio wave absorbingpanel must be made from several tens of cm to over a meter, andtherefore such panels cannot be realistically applied to ordinarywindows of buildings.

[0013] When a wireless LAN uses a frequency of 2.4 GHz, the twosubstrates need to be spaced about 31 mm apart. This makes thesubstrates inconvenient for use as building materials.

SUMMARY OF THE INVENTION

[0014] It is therefore an object of the present invention to provide aradio wave absorbing panel having superior radio wave absorbency andtransparency to light.

[0015] To achieve this and other objects, the invention provides a radiowave absorbing panel comprising at least two insulating substratesdisposed in parallel a predetermined distance apart on at least one sideof each of which is formed a continuous conducting film and at least oneinsulating substrate disposed between these insulating substrates inparallel therewith on a surface of which are formed conducting filmsdisposed in the form of stripes or in the form of a matrix.

[0016] As a result, in the invention, even when the relativepermittivity of the radio wave absorbing panel is made large and thethickness of the panel is made thin, it is possible to make itsabsorptance of radio waves high, and consequently the invention can beapplied for example to a panel for absorbing radio waves of VHF band andLAN frequency band highly suitable for installation in a window openingof a building.

[0017] In a radio wave absorbing panel according to the invention, whenstripe-form conducting films are used, preferably, the sheet resistivityof the continuous conducting film is from 1Ω/□ to 3000Ω/□ and when thewidth and the sheet resistivity of each of the conducting films disposedin the form of stripes are respectively written Hcm and R_(BM)Ω/□ andthe insulation resistance of the insulating substrate on which thestripe-form conducting films are formed is written R_(D)cmΩ, then H,R_(BM) and R_(D) are set in the ranges of: 0.1 cm≦H≦50 cm,1Ω/□≦R_(BM)≦40Ω/□, R_(D)≦ 30,000 cmΩ. when stripe-form conducting filmshaving these values are used, the relative permittivity of the radiowave absorbing panel can be made large and its absorptance of radiowaves raised, and even if the panel is made thin it is possible toimprove its absorbency of radio waves arriving from a fixed direction.

[0018] When matrix-form conducting films are used, preferably, the sheetresistivity of the continuous conducting film is from 1Ω/□ to 3000Ω/□and when the width, the length and the sheet resistivity of each of theconducting films disposed in the form of a matrix are respectivelywritten Hcm, Vcm and R_(BM)Ω/□ and the insulation resistance of theinsulating substrate on which the matrix-form conducting films areformed is written R_(D)cmΩ, then H, V, R_(BM) and R_(D) are set in theranges of: 0.1 cm≦H≦50 cm, 0.1 cm≦V≦50 cm, 1Ω/□≦R_(BM)≦40Ω/□. Whenmatrix-form conducting films having these values are used, the relativepermittivity of the radio wave absorbing panel can be made large and itsabsorptance of radio waves raised, and even if the panel is made thin itis possible to improve its absorbency of radio waves arriving from anydirection.

[0019] Also, when stripe-form conducting films are used, preferably, thesheet resistivity of the continuous conducting film formed on thesurface of one of the insulating substrates is not more than 30Ω/□ andthe sheet resistivity of the continuous conducting film formed on thesurface of another of the insulating substrates is from 50Ω/□ to 3000Ω/□and when the width and the sheet resistivity of each of the conductingfilms disposed in the form of stripes are respectively written Hcm andR_(BM)Ω/□ and the insulation resistance of the insulating substrate onwhich the conducting films disposed in the form of stripes are formed iswritten R_(D)cmΩ, then H, R_(BM) and R_(D) are made: 0.1 cm≦H≦ 50 cm,1Ω/□≦R_(BM)≦40Ω/□, R_(D)≧30,000 cmΩ. By making the sheet resistivity ofthe conducting film on one of the insulating substrates not more than30Ω/□ and the sheet resistivity of the conducting film of another of theinsulating substrates from 50Ω/□ to 3000Ω/□ in this way, it is possibleto make the panel still thinner while maintaining an ample radio waveabsorbency.

[0020] And when matrix-form conducting films are used, preferably, thesheet resistivity of the continuous conducting film formed on thesurface of one of the insulating substrates is not more than 30Ω/□ andthe sheet resistivity of the continuous conducting film formed on thesurface of another of the insulating substrates is from 50Ω/□ to 3000Ω/□and when the width, the length and the sheet resistivity of each of theconducting films disposed in the form of a matrix are respectivelywritten Hcm, Vcm and R_(BM)Ω/□ and the insulation resistance of theinsulating substrate on which the conducting films disposed in the formof a matrix are formed is written R_(D)cmΩ, then H, V, R_(BM) and R_(D)are set to: 0.1 cm≦H≦50 cm, 0.1 cm≦V≦50 cm, 1Ω/□≦R_(BM)≦ 40Ω/□,R_(D)≧30,000 cmΩ. By making the sheet resistivity of the conducting filmon one of the insulating substrates not more than 30Ω/□ and the sheetresistivity of the conducting film of another of the insulatingsubstrates from 50Ω/□ to 3000Ω/□ in this way, and because matrix-formconducting films set to predetermined values are being used, it ispossible to make the relative permittivity of the radio wave absorbingpanel large and raise its radio wave absorptance.

[0021] Preferably, transparent plate glass is used as the insulatingsubstrates and the stripe or matrix-form conducting films aretransparent conducting films composed mainly of SnO₂ or In₂O₃ or aremetal films composed mainly of Ag, Au, Cu or Al, whereby it is possibleto lower the sheet resistivity of the conducting films and raise theirtransmittivity of light.

[0022] Also, dry air may be preferably sealed in spaces between the atleast two insulating substrates and the at least one insulatingsubstrate on the surface of which are formed conducting films disposedin the form of stripes or a matrix, whereby condensation due to changesin outside temperature can be prevented and deterioration in radio waveabsorbing capacity due to water in the conducting films can beprevented.

[0023] Alternatively, resin may be preferably sealed in spaces betweenthe at least two insulating substrates and the insulating substrate onthe surface of which are formed conducting films disposed in the form ofstripes or a matrix to form a laminated glass structure, whereby it ispossible to prevent the glass from cracking and scattering.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Preferred embodiments of the present invention will now bedescribed in detail on the basis of the accompanying drawings, wherein:

[0025]FIG. 1 is a partially sectional view showing the construction of aradio wave absorbing panel according to the invention;

[0026]FIG. 2 is a pattern view showing conducting films according to theinvention disposed in the form of stripes;

[0027]FIG. 3 is a pattern view showing conducting films according to theinvention disposed in the form of a matrix;

[0028]FIG. 4A is a construction view of an insulating substrate on thesurface of which are formed stripe-form conducting films, FIG. 4B is aplan view of the stripe-form conducting films, and FIG. 4C is asectional view on the line C-C in FIG. 4B;

[0029]FIG. 5 is a radio wave transmission, reflection and absorptionfrequency characteristic chart for a case where an insulating substrateon which conducting films in the form of stripes are formed is notinterposed between two facing insulating substrates;

[0030]FIG. 6 is a radio wave transmission, reflection and absorptionfrequency characteristic chart for a case where an insulating substrateon which conducting films in the form of stripes are formed isinterposed between two insulating substrates according to the invention;

[0031]FIG. 7 is a graph showing theoretical calculation results obtainedfor a first preferred embodiment of the invention, Preferred Embodiment1; and

[0032]FIG. 8 is a graph showing theoretical calculation results obtainedfor a first comparison example of the invention, Comparison Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The present invention provides a thin radio wave absorbing panelwhich can be applied to window glass of a building and which providestransparency to visible light, prevents, by absorbing radio waves fromoutside, the reflection of radio waves and the transmission of radiowaves into the building, and reduces in-office radio waves.

[0034] In FIG. 1, a radio wave absorbing panel 1 is made up of a firstinsulating substrate 2 forming one side of the panel, a first conductingfilm 3 coated on the entire surface of the inner side of the firstinsulating substrate 2, a second insulating substrate 6 disposed inparallel with and a predetermined distance away from the firstinsulating substrate 2 and forming the other side of the panel, a secondinsulating film 7 coated on the entire surface of the inner side of thesecond insulating substrate 6, a middle insulating substrate 4 disposedbetween and in parallel with the first insulating substrate 2 and thesecond insulation substrate 6, and a conducting film 5 coated on thesurface of the middle insulating substrate 4 in the form of stripes orin the form of a matrix. The insulating substrates 2, 6, 4 are fixedlysecured and air layers 8 are sealed.

[0035] Although in the preferred embodiment shown in FIG. 1 the radiowave absorbing panel 1 has a three-layer structure made up of the firstinsulating substrate 2, the middle insulating substrate 4 and the secondinsulating substrate 6, the panel 1 may alternatively be made up of aplurality of such three-layer structures.

[0036] Also, the first conducting film 3 and the second insulating film7 may alternatively be coated continuously on the outer sides of therespective first and second insulating substrates 2 and 6, or may becoated continuously on both sides of the second insulating substrate 6.

[0037] Also, a plurality of middle insulating substrates 4 may bedisposed between the first insulating substrate 2 and the secondinsulating substrate 6.

[0038] Or, a single middle insulating substrate 4 and one or moreordinary insulating substrates may be disposed between the firstinsulating substrate 2 and the second insulating substrate 6.

[0039]FIG. 2 shows a pattern of stripe-form conducting films of a radiowave absorbing panel according to the invention.

[0040] As shown in FIG. 2, stripe-fore conducting films 5A have a stripewidth H and a gap D between adjacent stripes.

[0041]FIG. 3 shows a pattern of matrix-form conducting films of a radiowave absorbing panel according to the invention.

[0042] In FIG. 3, a matrix of conducting films 5B is made up of multiplerectangular film pieces arrayed in the form of a matrix. The width ofthe film pieces in their row and column directions are respectively Hand V, and the gap between pieces adjacent in the row and columndirections is D.

[0043]FIGS. 4A through 4C are views of an insulating substrate accordingto the invention made by forming stripe-shaped conducting films on asubstrate.

[0044]FIG. 4A shows the construction of a middle insulating substrate 4having stripe-form conducting films 5A formed on its surface and alsoillustrates the action of a uniform radio wave field impressed on thestripe-form conducting films 5A. FIG. 4B shows the pattern of thestripe-form conducting films 5A. FIG. 4C is a sectional view of thestripe-form conducting films 5A shown in FIG. 4B on the line C-C.

[0045] In FIG. 4A, the y-axis direction vector shows the direction ofthe uniform radio wave field and the vector having the x-z coordinateinclination α shows the propagation direction of the electromagneticwaves.

[0046] In FIG. 4C, the major axis of the flat ellipse (the width of thestripe-form film) of each of the stripe-form conducting films 5A incross-section is H, the minor axis (the thickness of the film) is A, andthe gap between adjacent stripe-form films is D. The insulationresistance between adjacent stripe-shaped films will be written R_(D),and the relative permittivity of the surroundings of the flat ellipticalfilms will be written _(ε)eX.

[0047] Generally, when two insulating substrates on an entire surface ofeach of which a conducting film has been formed are disposed in parallel(i.e. when in FIG. 1 there is no middle insulating substrate 4 butrather just an air layer 8), to produce radio wave absorptionaccompanying resonance caused by interference of multiple reflections,it is necessary to make the distance between the two substrates about¼of the wavelength of the radio waves. Therefore, because when the spacebetween the two substrates is an air layer its relative permittivity is1, with respect to radio waves of for example 100 MHz in frequency thedistance between the two substrates must be 75 cm. When glass, which hasa high relative permittivity, is used instead of air, because itsrelative permittivity is about 7, the distance between the twosubstrates becomes about 28 cm. To reduce the distance between the twosubstrates to about 10 cm, with respect to radio waves of frequency 100MHz, a medium having a relative permittivity of several tens or morebecomes necessary. When radio waves used in a wireless LAN has afrequency of 2.4 GHz and the layer between the two substrates is air,the distance between the two substrates must be about 31 mm.

[0048] Research carried out by the present inventors has shown that theconducting films 5A divided up in the form of stripes shown in FIG. 4Chave the property that with respect to a field of radio wavesperpendicular to the centerline of a gap between adjacent stripe-formfilms, under certain conditions, if the film thickness of thestripe-form conducting films 5A is written Acm and the major axis of theflat ellipse of the cross-sections of the films is written Hcm, theyhave a huge real relative permittivity of about H/A (in a normal caseabout 10⁷), and also can be regarded as a continuous medium of thicknessAcm.

[0049] A radio wave absorbing panel according to the invention has thesame function as when a dielectric film having this kind of huge realpermittivity is disposed between two insulating substrates disposed inparallel on each of which a continuous conducting film is formed, andcan effect a large phase change in radio waves across the conductingfilms divided up in the form of stripes or a matrix. As a result, evenwhen the distance between the two insulating substrates (the panelthickness) is much less than ¼of a wavelength, it is possible to produceresonance derived from interference of multiple reflections and therebyrealize a high radio wave absorbency.

[0050] Next, an approximate calculation of the above-mentioned huge realpermittivity will be carried out on the basis of a simple model.

[0051] Modeling was carried out by taking a stripe-form conducting film5A as a flat elliptical prism extending in a direction perpendicular tothe orientation of a radio wave field (the y-axis direction) as shown inFIG. 4C, forming the gaps between adjacent elliptical prisms as aninsulation resistance R_(D) and a distance (width) D, and surroundingthe elliptical prism with a dielectric of relative permittivity _(ε)eX;the polarization of a case wherein a uniform outside field is impressedon this one elliptical prism was computed, and by space-averaging overthe whole layer of the divided insulating films the effective relativepermittivity Σ_(BM) of this virtual dielectric layer is given by Exp.(1). $\begin{matrix}{\sum_{BM}{= {1 + \frac{{\pi \left( {H/\lambda} \right)}\left\lbrack {\left\{ {{ɛ^{eX}\kappa} + {\left( {ɛ^{eX} - 1} \right)\quad \left( {D \cdot {R_{BM}/R_{D}}} \right)}} \right\} - \left( {H \cdot {R_{BM}/R_{D}}} \right)} \right\rbrack}{\left( {\pi \quad {A/\lambda}} \right)\quad \left\{ {1 + {\left( {H/\lambda} \right)^{2}\left( {2\pi} \right)^{2}\left( {R_{BM}/Z_{o}} \right)^{2}}} \right\}} + {i\frac{\begin{matrix}{{\left( {H + D} \right)\quad \left( {{Z_{o}/2}R_{D}} \right)} +} \\{2\pi^{2}{ɛ^{eX}\left( {H/\lambda} \right)}^{2}\left( {\kappa + {D \cdot {R_{EM}/R_{p}}}} \right)\left( {R_{BM}/Z_{o}} \right)}\end{matrix}}{\left( {\pi \quad {A/\lambda}} \right)\left\{ {1 + {\left( {H/\lambda} \right)^{2}\left( {2\pi} \right)^{2}\left( {R_{BM}/Z_{o}} \right)^{2}}} \right\}}}}}} & (1)\end{matrix}$

[0052] where:

[0053] A is the minor axis of the elliptical prism (equivalent to theconducting film thickness);

[0054] H is the major axis of the elliptical prism (equivalent to thewidth of the stripe-form conducting films);

[0055] D is the width of the high-resistance part dividing adjacentstripe-form films;

[0056] k is the coverage, (π/4)AH/{A(H+D)};

[0057] λ is the wavelength [cm] in a vacuum of the incident radio waves;

[0058] Z₀ is the space impedance of a vacuum, 4πc/109≈377Ω;

[0059] R_(BM) is the sheet resistivity of the stripe-form film [Ω/□ ];and

[0060] R_(D) is the insulation resistance between adjacent stripe-formfilms of unit length [cmΩ].

[0061] When the insulation resistance R_(D) of the gaps between thestripe-form films is sufficiently large, by taking R_(D) as infinity,Exp. (1) can be simplified to Exp. (2). $\begin{matrix}\begin{matrix}{\sum_{BM}{= \quad {1 + \frac{ɛ^{eX}\left( {H/A} \right)}{1 + {\left( {H/\lambda} \right)^{2}\left( {2\pi} \right)^{2}\left( {R_{BM}/Z_{o}} \right)^{2}}} +}}} \\{\quad {2\pi \quad i\frac{ɛ^{eX}\kappa \quad \left( {H/A} \right)\quad \left( {H/\lambda} \right)\quad \left( {R_{BM}/Z_{o}} \right)^{2}}{1 + {\left( {H/\lambda} \right)^{2}\left( {2\pi} \right)^{2}\left( {R_{BM}/Z_{o}} \right)^{2}}}}}\end{matrix} & (2)\end{matrix}$

[0062] Also, when the major axis H of the elliptical conducting films isset to a value sufficiently smaller than the wavelength λ of theincident radio waves (H<<λ) and the sheet resistivity R_(BM) of theconducting films is set to a value sufficiently smaller than the spaceimpedance of a vacuum Z₀ (R_(BM)<<Z₀), Exp. (2) can be simplified toExp. (3).

Σ_(BM)≈_(ε)eX_(k)(H/A) . . .   (3)

[0063] From Exp. (3), the effective relative permittivity Σ_(BM) of thevirtual dielectric layer of the stripe-form conducting films 5A shown inFIGS. 4A through 4C is equivalent to a huge real relative permittivity(about 10⁷).

[0064] Using the relative permittivity Σ_(BM) shown in Exp. (3) , theradio wave transmission, reflection and absorption characteristics ofwhen with two insulating substrates on each of which is formed acontinuous conducting film disposed in parallel an insulating substrateon which is formed a conducting film divided up into stripes isinterposed therebetween were calculated using Maxwell's equations.

[0065] Calculated results of radio wave transmission, reflection andabsorption frequency characteristics for when with two insulatingsubstrates on each of which is formed a continuous conducting filmdisposed in parallel an insulating substrate on which is formed aconducting film divided up into stripes is not and is interposedtherebetween are shown in FIG. 5 and FIG. 6, respectively.

[0066]FIG. 5 shows radio wave transmission, reflection and absorptionfrequency characteristics of when an insulating substrate on which isformed a conducting film divided up into stripes is not provided. Inthis case, the thickness of the air layer between the two insulatingsubstrates on each of which a continuous conducting film is formed istaken as 60 cm.

[0067] In FIG. 5, in the vicinity of the frequency 100 MHz, the radiowave absorptance is very high (absorptance=1), while the radio wavereflectivity decreases greatly (reflectivity= 0.001; attenuation 60 dB).

[0068]FIG. 6 shows radio wave transmission; reflection and absorptionfrequency characteristics of when a middle insulating substrate on whichis formed a conducting film divided up into stripes according to theinvention is interposed between first and second insulating substrates.The gap between the first insulating substrate 2 and the middleinsulating substrate 4 shown in FIG. 1 was taken as 15 cm and the gapbetween the middle insulating substrate 4 and the second insulatingsubstrate 6 as 1 cm.

[0069] In FIG. 6, in the vicinity of the frequency 100 MHz, the radiowave absorptance is very high (absorptance=1), while the radio wavereflectivity decreases greatly (reflectivity= 0.01; attenuation 40 dB).

[0070] In this way, in a radio wave absorbing panel according to theinvention, because a middle insulating substrate on which is formed aconducting film divided up into stripes is provided between at least twoinsulating substrates on each of which a continuous conducting film isformed, even when the panel width is reduced to ¼compared to when thereis no middle insulating substrate on which is formed a conducting filmdivided up into stripes, the radio wave reflectivity can be reduced andthe absorptance increased

[0071] In the characteristics of the continuous conducting films used inthe radio wave absorbing panel of the invention there is an action ofreflecting nearly 100% of radio waves and an action of reflecting ortransmitting some radio waves and absorbing some.

[0072] To obtain the former characteristic, a conducting film having asmall sheet resistivity R_(BM) is suitable, but to make the sheetresistivity R_(BM) small it is necessary to make the conducting filmthick, and its manufacturing cost increases. Also, it a metal film isused as the conducting film, when the conducting film is made thick itstransparency to visible light decreases. For this reason, to eliminateproblems of manufacturing cost and visible light transparency, the sheetresistivity R_(BM) is set to between 1Ω/□ and 3000Ω/□. Most preferably,the sheet resistivity R_(BM) is set to between 5Ω/□ and 20Ω/□ so thatvisible light transparency which is not much different from that ofordinary transparent glass can be obtained.

[0073] In the case of the latter characteristic, when multipleinsulating substrates coated with continuous conducting films are usedthe radio wave absorption characteristics variously change, and when thesheet resistivity R_(BM) is made greater than 3000Ω/□ there is too muchtransmission of radio waves and it is difficult to obtain sufficientabsorption, and when on the other hand the sheet resistivity R_(BM) ismade less than 5Ω/□ it becomes difficult to obtain sufficient absorptionbecause there is too much radio wave reflection. Therefore, the sheetresistivity R_(BM) is set preferably between 200 6/□ and 1500Ω/□.

[0074] In the characteristics of the stripe-form or matrix-formconducting films, to raise the radio wave absorptivity the effectiverelative permittivity Σ_(BM) must be made large. When the real part ofthe relative permittivity Σ_(BM) is not sufficiently large compared withthe imaginary part, resonance does not readily occur and the absorptancefalls, and therefore it is important for the major axis H of theelliptical conducting films to be set to a value sufficiently smallerthan the wavelength λ of the incident radio waves (H<<λ) and for thesheet resistivity of the conducting films to be set sufficiently smallerthan the space impedance of a vacuum Z₀ (R_(BM)<<Z₀).

[0075] From Exp. (3), the relative permittivity Σ_(BM) can be set to alarge value when the value of the width H of the stripe-form conductingfilms is made large, but because the wavelength λ of the arriving radiowaves when they are in the VHF band is about 300 cm, to obtain H<<λ thewidth H of the stripe-form conducting films is made smaller than 50 cm.When the frequency used for a wireless LAN is considered and the width Hof each stripe-form conducting film is made small, the relativepermittivity Σ_(BM) becomes small, whereby the radio wave absorptancebecomes small, and also the conducting films become difficult tomanufacture. For this reason, the value of H is normally set to at least0.1 cm (H≧0.1 cm). Sated otherwise, the width H of each stripe-formconducting film falls in the range of 0.1 cm≦H≦50 cm when λ is in therange of 1 cm-300 cm. λ used herein represents a wave length of a radiowave in a vacuum.

[0076] For the same reason as in the stripe-form conducting film, therow-direction pattern width V of each matrix-form conducting film shownin FIG. 3 is also in the range of 0.1 cm≦ V≦50 cm.

[0077] The sheet resistivity R_(BM) of the stripe-form conducting filmsmust as mentioned above be made a value sufficiently smaller than thespace impedance of a vacuum Z₀ (R_(BM)<<Z₀≈ 377Ω), and normally thesheet resistivity R_(BM) is set to not more than 40Ω/□ (R_(BM) ²40Ω/□).

[0078] However, to reduce the sheet resistivity R_(BM) it is necessaryto increase the thickness of the conducting films and theirmanufacturing cost increases. When the film thickness of the conductingfilms and metal films are used as the conducting films, there arises theproblem that their transmittivity of visible light decreases. For thisreason the sheet resistivity R_(BM) is set to at least 1Ω/□(R_(BM)≧1Ω/□) and preferably from 5Ω/□ to 20Ω/□.

[0079] In the conditions for the real part of the relative permittivityΣ_(BM) being larger than the imaginary part, it is necessary for theinsulation resistance R_(D) of the gap between adjacent stripe-formfilms to be large, as shown in Exp. (4).

π(H/λ)_(ε)eX_(k)>>(H+D)(Z₀/2R_(D))  (4)

[0080] Normally, the width H of the stripe-form conducting films isgenerally set amply larger than adjacent the stripe-form film gap D(H>>D), and applying this condition to Exp. (4) yields Exp. (5).

R_(D)>>2λZ₀/(π²ε^(eX))  (5)

[0081] When radio wave wavelength λ=300 cm, relative permittivityε^(eX)=7 (glass), and Z₀=377Ω are substituted into Exp. (5), insulationresistance of stripe-form film gap R_(D)>>3300 cmΩ is obtained, andnormally the insulation resistance R_(D) is set greater than 30,000 cmΩ.

[0082] When on a conducting film divided up in one direction in the formof stripes a field of radio waves is impressed in the length directionof the stripes, the effect of the stripe-form conducting films discussedabove cannot be expected, and the same characteristics as those of acontinuous conducting film are obtained.

[0083] To obtain the same radio wave absorbing characteristics asstripe-form conducting films with respect to an electric field of anyorientation, the conducting films are set to a matrix form. In thematrix shape, the conditions of the unidirectional stripe shapementioned above must be satisfied.

[0084] Next, as a preferred embodiment of a radio wave absorbing panelaccording to the invention, a panel made by forming on one of theinsulating substrates constituting the panel shown in FIG. 1 aconducting film having an action of reflecting nearly 100% of radiowaves and forming on the other of the insulating substrates a conductingfilm having the action of partially reflecting and transmitting andpartially absorbing radio waves and disposing between these twosubstrates at least one insulating substrate on a surface of which areformed matrix-form conducting films will be described.

[0085] In this kind of radio wave absorbing panel, compared with a casewherein an insulating substrate having the function of partiallyreflecting and transmitting and partially absorbing radio waves isdisposed over the entire face of the outer side of the panel, thethickness of the air layer can be reduced to about ½, and also theeffect of the insulating substrate on which the stripe-form ormatrix-form conducting films are formed can be made the equivalent ofthe effect of double the number of such substrates.

[0086] The sheet resistivity R_(BM) of the continuous conducting filmhaving the function of reflecting nearly 100% of radio waves is set to1Ω/□ to 30Ω/□. A particularly optimal sheet resistivity R_(BM) is 5Ω/□to 20Ω/□. However, when transparency to light is not required, aconducting film having a lower sheet resistivity R_(BM) can be used anda more marked effect can thereby be obtained.

[0087] The sheet resistivity R_(BM) of the continuous conducting filmhaving the function of partially reflecting and transmitting andpartially absorbing radio waves is set to 50Ω/□ to 3000Ω/□. Aparticularly optimal sheet resistivity R_(BM) is 200Ω/□ to 1500Ω/□.

[0088] To apply a radio wave absorbing panel according to the inventionto a window opening of a building it is necessary to use substrateswhich are transparent to visible light as the insulating substrates, andtransparent plate glass is suitable. Also for the conducting filmsformed on the insulating substrates, conducting films transparent tolight are used. In particular, to make the sheet resistivity of thestripe-form or matrix-form conducting films low and also maintaintransparency to light, the film material must be selected from a limitedrange of alternatives. As film, materials with which this kind offunction can be realized, for example transparent conducting filmscomposed mainly of SnO₂ or In₂O₃ or metal films composed mainly of Ag,Au, Cu or Al are suitable.

[0089] Because these conducting films or metal films reflectnear-infrared rays of sunlight and have a low heat ray radiationfunction, in addition to their radio wave absorption function, when thepanel is used as window glass, they enable energy of roomair-conditioning to be saved.

[0090] When these film materials, and particularly metal films mainlycomposed of either Ag, Au, Cu or Al are used, to ensure durability ofthe films, multiple layer glass made by sealing dry air between glasssubstrates, or laminated glass made by inserting resin between glasssubstrates, is preferably used. To impart a radio wave absorbing actionusing dry air or resin, the thicknesses of the air layers, thethicknesses of the glass substrates and the thicknesses of the resinlayers are set on the basis of the conditions discussed above.

[0091] Preferred embodiments of the invention will be describedspecifically along with comparison examples.

[0092] First Preferred Embodiment

[0093] Using an in-line sputtering apparatus, a gas of 90 mol %nitrogen, 10 mol % oxygen was introduced and by reactive sputteringusing a Cr metal target an oxide nitride CrOxNy film was formed on asheet of soda lime silica glass of thickness 4 mm. The thickness of thisfilm was about 40 nm and the sheet resistivity of the film was about400Ω/□.

[0094] Next, by means of an in-line sputtering apparatus, on a soda limesilica glass substrate of thickness 4 mm, films of ZnO, Ag and ZnO wereformed in order from the substrate side as an Ag film. The in-linesputtering apparatus was controlled so that the respective filmthicknesses were 40 nm, 15 nm and 40 nm. The ZnO films have the role ofmaintaining the durability of the Ag film. The sheet resistivity of thefilm obtained was about 5Ω/□.

[0095] Also, using the same in-line sputtering apparatus, a thinstainless steel plate mask of thickness about 0.5 mm having square holesof side length 5 cm formed therein with gaps of 4 mm between adjacentholes was prepared and placed on a soda lime silica glass substrate ofthickness 4 mm, and films of ZnO, Ag and ZnO were formed in order fromthe glass substrate side with the in-line sputtering apparatus beingcontrolled so that the film thicknesses were respectively 40 nm, 15 nmand 40 nm.

[0096] The mask was then removed to form 5 cm-square transparentZnO/Ag/ZnO films on the substrate in the form of a matrix. The sheetresistivity of the films was about 5Ω/□ and the insulation resistancebetween adjacent films was over 20MΩ.

[0097] These three glass substrates were then cut into squares of side120 cm and used to construct a laminate of the structure: 4 mmglass/CrOxNy. 0.6 cm air layer: matrix of ZnO/Ag/ZnO films/4 mm glass:2.4 cm air layer: ZnO/Ag/ZnO film/4 mm glass.

[0098] When the radio wave reflectivity and the radio wavetransmittivity from 200 MHz to 1 GHz of this laminate were measured, itwas found that at around 500 MHz the reflectively decreased and therewas a region where strong resonant absorption occurred.

[0099] Computed results of radio wave reflectivity and absorptioncharacteristics obtained using the approximate theory discussed abovefor this panel are shown in FIG. 7.

[0100] First Comparison Example

[0101] A laminate was made by replacing the glass substrate of thickness4 mm having a matrix-form Ag film formed thereon of the first preferredembodiment with a glass substrate having no film formed on it, and theradio wave reflectivity and the radio wave transmittivity from 200 MHzto 1 GHz of this laminate were measured. The result was that althoughthere was a decrease in reflectivity and an increase in transmittivityat around 1 GHz, at below 1 GHz there was no resonant absorption.

[0102] In theoretically computed results of radio wave reflectivity andtransmittivity obtained for this construction, as shown in FIG. 8 therewas resonant absorption at around 1.2 GHz.

[0103] To induce resonant absorption at around the same frequency of 500MHz as the first preferred embodiment, it was necessary to increase thethickness of the 2.5 cm air layer to 10 cm.

[0104] Second Preferred Embodiment

[0105] An indium tin oxide (ITO) sintered target was mounted in anin-line sputtering apparatus and a gas made by adding 20 mol % oxygen toAr was introduced to form an ITO film of thickness about 60 nm on a sodalime silica glass substrate of thickness 4 mm. The film-formingconditions were controlled so as to bring the sheet resistivity of thefilm to about 400Ω/□.

[0106] A laminate was made by substituting this substrate for thesubstrate having the CrOxNy film formed thereon of the first preferredembodiment and the radio wave reflectivity and transmittivity from 200MHz to 1 GHz of this laminate were measured. The result was that thelaminate exhibited substantially the same characteristics as the firstpreferred embodiment.

[0107] Third Preferred Embodiment

[0108] Using an in-line sputtering apparatus, SnO₂, Ag and SnO₂ filmswere formed on a soda lime silica glass substrate of thickness 4 mm. Thefilm thicknesses were respectively controlled to 40 nm, 15 nm and 40 nm,and a film of sheet resistivity 5Ω/□ was obtained.

[0109] The film face of this glass substrate was scored in acheckerboard pattern using a steel needle and thereby divided up intosquares of side length about 20 cm. When the divisions were viewed withan optical microscope the film was seen to have been cut away in linesof just under 200 μm in width. The electrical resistance betweenadjacent conducting films thus divided was generally over about 50kΩ. Alaminate was made by substituting this glass substrate with films in theform of a matrix of 20 cm squares formed thereon for the glass substratehaving the matrix of ZnO/Ag/ZnO films formed thereon of the firstpreferred embodiment, and the radio wave reflectivity and transmittivityfrom 200 MHz to 1 GHz of this laminate were measured. The result wasthat a slightly weaker resonant absorption than that in the firstpreferred embodiment occurred at around 250 MHz.

[0110] Second Comparison Example

[0111] Using an in-line sputtering apparatus, ITO, Ag and ITO films wereformed on a glass substrate. The film thicknesses were respectivelycontrolled to 40 nm, 15 nm and 40 nm, and a film of sheet resistivityabout 5Ω/□ was obtained.

[0112] In the same way as in the third preferred embodiment, the filmface was scored in a checkerboard pattern using a steel needle andthereby divided up into squares of side length about 20 cm. Theelectrical resistance between adjacent conducting films thus divided wasless than 50Ω (about 1000 cmΩ as R_(D)).

[0113] A laminate was made by substituting this glass substrate withfilms in the form of a matrix of 20 cm squares formed thereon for theglass substrate having the matrix of ZnO/Ag/ZnO films formed thereon ofthe first preferred embodiment, and the radio wave reflectivity andtransmittivity from 200 MHz to 1 GHz of this laminate were measured. Theresult was that although there was a gentle decrease of reflectivityaround frequency 250 MHz, resonant absorption did not occur.

[0114] Fourth Preferred Embodiment

[0115] The Cr oxide nitride film CrNxOy of sheet resistivity about400Ω/□ mentioned in the first preferred embodiment was formed on glassof thickness 10 mm and this was cut into a square of side length 80 cmand stood perpendicular to a floor surface.

[0116] Cubes of foam styrol of side length 1 cm were then affixed to thefilm face side of this glass as spacers at lattice points about 30 cmapart.

[0117] Colorless float glass of plate thickness 4 mm coated with atransparent conducting film consisting of an SnO₂ film of sheetresistivity about 10Ω/□ doped with fluorine of film thickness about 300nm was cut into strips of width 20 cm, length 180 cm, and these werelined up closely in the longitudinal direction on a plate of plastic andfixed to make a panel of side length 180 cm.

[0118] This panel was stood vertical and placed against the 10 mm-thickglass with the foam styrol spacers therebetween.

[0119] Also, so as to sandwich this panel and parallel therewith anotherpanel having aluminum foil of thickness 15 μm stretched over it was sodisposed that the thickness of an air layer formed between the panelscould be varied.

[0120] Linearly polarized radio waves were then directed at the laminatethus constructed from the 10 mm-thick glass side, and its reflectivityin a frequency range of 50 MHz to 500 MHz was measured.

[0121] When the polarization of the radio wave field was horizontal andthe thickness of the variable air layer was about 15 cm, at about 100MHz there was a marked fall in reflectivity and strong resonantabsorption occurred.

[0122] Third Comparison Example

[0123] When radio waves whose field was polarized vertically weredirected at the laminate construction of the fourth preferredembodiment, the reflectivity in the frequency range of 50 MHz to 500 MHzwas substantially 100%.

[0124] Fourth Comparison Example

[0125] From the laminate construction of the fourth preferredembodiment, the panel made by lining up 20 cm×180 cm pieces of glasscoated with transparent conducting film was removed and the thickness ofthe air layer was varied to investigate the condition under whichresonant absorption with respect to horizontally polarized waves aroseat around 100 MHz. It was found that a thickness of the air layer ofabout 70 cm was necessary.

[0126] Fifth Preferred Embodiment

[0127] A soda lime silica glass substrate of thickness 4 mm coated witha continuous ITO film of sheet resistivity about 400Ω/□, a soda limesilica glass substrate of thickness 18 mm coated with transparentconducting films made by forming an SnO₂/Ag/SnO₂ film of sheetresistivity about 5Ω/□ and then dividing this up into a matrix ofsquares ox side length 5 cm by scoring with a steel needle, and a sodalime silica glass substrate of thickness 4 mm coated with a continuousZnO/Ag/ZnO/Ag/ZnO film of sheet resistivity about 3Ω/□ were laminatedtogether with resin interposed therebetween to form a laminate panel ofthe construction. 4 mm glass/ITO film 0.36 mm resin: dividedSnO₂/Ag/SnO₂ film/18 mm glass: 0.36 mm resin: ZnO/Ag/ZnO/Ag/ZnO film/4mm glass.

[0128] Radio waves were directed at this laminate panel from the side ofthe glass substrate coated with the ITO film, and its radio wavereflectivity and radio wave transmittivity from 400 MHz to 1.5 GHz weremeasured. It was found that resonant absorption occurred at around 600MHz.

[0129] Fifth Comparison Example

[0130] A laminate glass was constructed by using a soda lime silicaglass substrate of thickness 18 mm not coated with anything in place ofthe soda lime silica glass substrate of thickness 18 mm on which thematrix of transparent conducting films was formed in the laminateconstruction of the fifth preferred embodiment.

[0131] When radio wave were directed at this laminate panel from theside of the glass substrate coated with the ITO film and the radio wavereflectivity and radio wave transmittivity from 400 MHz to 1.5 GHz weremeasured, resonant absorption occurred at around 1.2 GHz.

Industrial Applicability

[0132] As described above, with this invention it is possible toincrease the relative permittivity of a radio wave absorbing panel andobtain a thin radio wave absorbing panel having excellent radio waveabsorptance as well as excellent transparency to light. Thus, a radiowave absorbing panel according to the invention is ideal as a panel forinstallation in window openings of buildings and as a panel forabsorbing high frequency radio saves generated in offices.

What is claimed is:
 1. A radio wave absorbing panel comprising: at leasttwo insulating substrates disposed in parallel a predetermined distanceapart on at least one side of each of which is formed a continuousconducting film; and at least one insulating substrate disposed betweenthese insulating-substrates in parallel therewith on a surface of whichare formed conducting films disposed in the form of stripes or in theform of a matrix.
 2. A radio wave absorbing panel according to claim 1 ,wherein the sheet resistivity of the continuous conducting film is inthe range of 1Ω/□ to 3000Ω/□ and when the width and the sheetresistivity of each of the conducting films disposed in the form ofstripes are respectively written Hcm and R_(BM)Ω/□ and the insulationresistance of the insulating substrate on which the conducting filmsdisposed in the form of stripes are formed is written R_(D)cmΩ, then H,R_(BM) and R_(D) have the following values: 0.1 cm≦H≦50 cm1Ω/□≦R_(BM)≦40Ω/□ R_(D)≧30,000 cmΩ.
 3. A radio wave absorbing panelaccording to claim 1 , wherein the sheet resistivity of the continuousconducting film is in the range of 1Ω/□ to 300Ω/□, and when the width,the length and the sheet resistivity of each of the conducting filmsdisposed in the form of a matrix are respectively written Hcm, vcm andR_(BM)Ω/□ and the insulation resistance of the insulating substrate onwhich the conducting films disposed in the form of a matrix are formedis written R_(D)cm, then H, V, R_(BM) and R_(D) have the followingvalues: 0.1 cm≦H≦50 cm 0.1 cm≦V≦50 cm 1Ω/□≦R_(BM)≦40Ω/□. R_(D)≧30,000cmΩ.
 4. A radio wave absorbing panel according to claim 1 , wherein thesheet resistivity of the continuous conducting film formed on thesurface of one of the insulating substrates is in the range of 1Ω/□ to30Ω/□ and the sheet resistivity of the continuous conducting film formedon the surface of another of the insulating substrates is in the rangeof 50Ω/□ to 3000Ω/□, and when the width and the sheet resistivity ofeach of the conducting films disposed in the form of stripes arerespectively written Hcm and R_(BM)Ω/□ and the insulation resistance ofthe insulating substrate on which the conducting films disposed in theform of stripes are formed is written R_(D)cmΩ, then H, R_(BM) and R_(D)have the following values: 0.1 cm≦H≦50 cm 1Ω/□≦R_(BM)≦40Ω/□ R_(D)≧30,00cmΩ.
 5. A radio wave absorbing panel according to claim 1 , wherein thesheet resistivity of the continuous conducting film formed on thesurface of one of the insulating substrates is in the range of 1Ω/□ to30Ω/□ and the sheet resistivity of the continuous conducting film formedon the surface of another of the insulating substrates is in the rangeof 50Ω/□ to 3000Ω/□, and when the width, the length and the sheetresistivity of each of the conducting films disposed in the form of amatrix are respectively written Hcm, Vcm and R_(BM)Ω/□ and theinsulation resistance of the insulating substrate on which theconducting films disposed in the form of a matrix are formed is writtenR_(D)cmΩ, then H, V, R_(BM) and R_(D) have the following values: 0.1cm≦H≦50 cm 0.1 cm≦V≦50 cm 1Ω/□≦R_(BM)≦40Ω/□ R_(D)≧30,000 cm Ω.
 6. Aradio wave absorbing panel according to claim 1 , wherein transparentplate glass is used as the insulating substrates and the stripe ormatrix form conducting films are transparent conducting films composedmainly of SnO₂ or In₂O₃ or are metal films composed mainly of Ag, Au, Cuor Al.
 7. A radio wave absorbing panel according to claim 1 , whereindry air is sealed in spaces between the at least two insulatingsubstrates and the at least one insulating substrate on the surface ofwhich are formed conducting films disposed in the form of stripes or amatrix.
 8. A radio wave absorbing panel according to claim 1 , whereinresin is sealed in spaces between the at least two insulating substratesand the insulating substrate on the surface of which are formedconducting films disposed in the form of stripes or a matrix to form alaminated glass structure.